Fuel nozzles for gas turbine engines on an aircraft direct fuel from a manifold to a combustion chamber. The fuel nozzle typically has an inlet fitting connected to the manifold, a spray nozzle located within the combustion chamber of the engine for atomizing the fuel, and a stem extending between and fluidly interconnecting the inlet fitting and the atomizing nozzle. Appropriate check valves and/or flow dividers can be disposed within the fuel nozzle to control the flow of fuel through the spray nozzle. Each fuel nozzle has an attachment flange which enables the nozzles to be attached to the combustor casing of the engine in a spaced-apart manner to dispense fuel in a generally cylindrical pattern.
The fuel nozzle typically includes a heat shield assembly surrounding the portion of the stem within the engine casing. The heat shield assembly is necessary because of the high temperatures within the engine casing. The heat shield assembly prevents the fuel from breaking down into its constituent components (i.e., "coking") which occurs when the wetted walls of a fuel passage exceed 400 degrees Fahrenheit. The coke in the fuel nozzle can build up to restrict fuel flow through the nozzle.
The heat shield assembly typically comprises a pair of outer U-shaped heat shield members which are located on opposite sides of the nozzle stem, and extend axially therealong. The heat shield members are secured together along opposed abutting surfaces such as by welding or brazing the seams between the heat shield members. The heat shield assembly is then attached to the fuel nozzle. In the past, it is known that the open upper end of the heat shield assembly has been attached to an enlarged neck on the stem of the fuel nozzle. The neck of the stem is located directly below the attachment flange for the fuel nozzle. Known techniques for attaching the heat shield assembly to the neck include brazing, welding, or mechanical means such as by clamping or friction fit. The braze or weld is provided entirely around the neck of the stem, while the clamps typically surround the upper end of the heat shield members. The attachment of the shield assembly to the nozzle also closes off the upper end of the heat shield assembly to prevent heated and pressurized gases from flowing between the heat shield assembly and the stem of the fuel nozzle. The heat shield members are typically unattached at their bottom end around the atomizing nozzle to allow for thermal expansion of the heat shield assembly.
The known attachment techniques for the heat shield assembly are not without drawbacks. For example, there can be large stresses associated with fixedly attaching the upper end of the heat shield assembly to the neck of the fuel nozzle. These stresses occur because of the operating conditions of the engine (high temperatures, transients, etc.), as well as because of the difference in material and dimensional characteristics of the heat shield members and the fuel nozzle. In particular, the heat shield members are much thinner and are typically formed from a more flexible and workable material than the fuel nozzle. The thermal expansion characteristics of the heat shield assembly can differ greatly from the fuel nozzle. A thermal gradient can therefore appear across the braze or weld attachment of the heat shield members to the fuel nozzle, which can detrimentally affect this attachment over time. The stresses can also loosen or break clamps or other mechanical means holding the heat shield assembly on the fuel nozzle.
Thus, it is believed that there is a demand in the industry for a heat shield assembly which is securely and reliably attached to the fuel nozzle of a gas turbine engine.